You can also rely on some cheap regulators
to be very accurate. We are using a Microchip (sorry!) MCP1700 which is
available in many different voltages, very low dropout, VERY low quiescent
current (1uA) and typical 0.4% accuracy as our power regulator. Microchip
quote a $0.35 'budget' price. Good for 'always on' battery equipment. If
you do one measurement with VCCA as a reference and then with the internal
reference you can get a correction factor if you need very accurate measurements
on the internal reference. This is useful when you only need a slow infrequent
accurate measurement such us where we need an accurate raw battery voltage
and a correction factor calculation time is insignificant. Since the unit
can go on working with the regulator in dropout we store the correction
factor when the battery voltage is high and go on using it after we cannot
rely on VCCA being 3.0V. Since the bandgap reference in the mega88 is relatively
unaffected by VCCA dropping out we can still measure the battery voltage.
Dave Pallant
AVR-Chat@yahoogroups.com wrote on 23/08/2005 10:46:15:
> David Kelly wrote:
>
> > Am a bit disappointed in the accuracy of the internal 2.56 volt
> > reference used by the A/D converter. Have a nice 3.3 volts on
the chip
> > supply. A 0.1 uF ceramic cap across AREF and GND. Nothing connected
to
> > AVCC.
> >
> > Working the math backwards agrees with the voltage measured across
the
> > 0.1 uF capacitor, that my reference voltage varies from 2.44
to 2.60 on
> > different units.
> >
> > Is this typical?
> >
>
> VERY, I am yet to find a "good" reference on a uC, but as
far as cheap
> ones go, I am using the ref2940 (IIRC) from TI, they are cheap, neat
and
> in a 3 pin sot23, take up little space and work well :)
>
> Regards,
>
> Kat.
>
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